Medical X-ray transparencies usually are examined by placing them over the display area of a device commonly referred to as an illuminator. Conventional illuminators normally comprise a box-like structure enclosing fluorescent lighting tubes behind a semi-transparent light diffusing display plate defining the display area. Commonly, transparencies are retained on the surface of the display plate by pushing the upper edge of the transparencies under spring-loaded film-holder clips located along the top edge of the display plate.
Standard size illuminators have a display plate 17 inches high and 14 inches or multiples of 14 inches (i.e. 28 inches or 56 inches) wide. Usually, each 14 inch width of display plate has its own fluorescent tubes and control switch. Such display plates enable viewing full size X-ray films which measure 17 inches by 14 inches. In such cases, the sections of the display plate not covered by transparencies need not be illuminated. This eliminates unnecessary glare from areas outside the transparency.
When transparencies smaller than 14 inches by 17 inches are to be examined, they are typically retained on the display area in the same manner as full size transparencies, i.e., suspending them by means of the film-holders along the top of the viewer. This leaves a portion of the display area surrounding the transparencies fully illuminated and the resulting additional glare detracts from the visual perception of the person trying to study the transparency and assess the information it contains.
Often, transparencies contain several very transparent areas, and frequently, radiologists have to examine over-exposed transparencies. In these cases, considerable glare emanates through areas of the transparencies themselves.
An important factor in the interpretation of transparencies, is the ability to discriminate between various levels of light. This ability is determined by the Weber Law. E. H. Weber found that "the minimum perceptible difference in a stimulus is proportional to the level of the stimulus". Stated in terms of vision, as formulated by Fechner, .delta.L/L=K (Weber constant); Where .delta.L is the minimal detectable difference in luminance; and L is the luminance, see "Elemente der Psychophysic", G. T. Fechner, Leipzig, 1860, and "Visual Psychophysics", D. Jameson and L. M. Hurvich (ed.), Berlin, 1972.
Accordingly, if the eye is adapted to luminance L, .delta.L is determined. For a radiologist, maximal gray level discrimination is desired. Therefore, the observer's eye should be adapted to the luminance level of the image under study. In less benign conditions, a person reading an X-ray will be less able to perceive critical but minor shadings and nuances in the transparency. Moreover, protracted inspection of display areas under less benign conditions involves significant eye strain on the part of the observer.
While it is of course feasible for an observer to overlay masking strips on the display area and thus block unwanted and contrast-reducing light passing through the diffuser, as a practical matter, readers of X-rays rarely resort to such practice.
Attempts have been made in the past to provide viewing devices for X-ray transparencies which shield the eyes of the observer from light emanating from light sources other than the light passing through the transparencies, to obscure light in parts of the transparencies, and to reduce the contrast in transparencies when so required. However, these devices do not adapt the level of light to the transparency or to the sections being studied in the transparency.
In U.S. Pat. No. 1,988,654 to Haag, there is disclosed a light box which incorporates two manually movable curtains for masking all of the light transmitting surfaces of a diffuser up to the edges of a transparency.
U.S. Pat. No. 2,436,162 to Cadenas discloses an X-ray viewer having a masking arrangement incorporating a plurality of hingedly connected opaque masks which may be manually pivoted relative to each other to expose all or only selected parts of an X-ray transparency.
U.S. Pat. No. 4,004,360 to Hammond is directed to a self-masking viewing device which purports to automatically obscure areas of the viewing screen not occupied by the X-ray transparency. In accordance with such device, the screen is provided with a multiplicity of holes which may be selectively blocked by shutters or opened for the passage of light. The interior of the device is connected to a vacuum source which functions to hold the film against the front surface of the device.
The vacuum functions, in addition, to close the shutters connected with those of the holes not covered by the transparencies, so that passage of light through such holes is prevented. Air cannot pass through those of the holes in registry with the transparencies and, thus, the shutters associated with the covered holes remain open for the passage of light. The device described is unsuitable for critical X-ray transparencies inspection since the presence of holes and shutters in the viewing screen in the areas in registry with the transparencies creates a pattern behind the transparencies which interferes with the ability to accurately read them.
U.S. Pat. No. 4,373,280 to Armfield discloses an X-ray viewing plate having a cross bar for supporting transparencies at a central portion of the screen. A series of shades is provided which may be manually activated to obscure selected parts of the illuminated surface.
U.S. Pat. No. 4,510,708 to Porkinchak discloses an X-ray viewing device which includes a series of masks on an elongated scroll. In a specific embodiment, the scroll is moved by a motor on a pair of feed rolls. The masks are sized to correspond with stock sizes of X-ray transparencies. The apparatus has a dimensional sensing mechanism which aligns a selected mask with a positioned transparency automatically in accordance with the sensed dimension. The transparencies are inserted into a film-holder. The widthwise sensing function is performed by a series of levers or fingers positioned to engage an edge of the film.
U.S. Pat. No. 4,637,150 to Geluk describes a system in which a cathode ray tube is used as a light source and the light is light emitted by this source is modulated in accordance with the stored density of a a transparency. This system is impractical due to the limited sizes and associated light outputs for this type of illuminator.